


At His Mercy

by SynfulCynical



Category: Undertale (Video Game)
Genre: Begging, Drug-Induced Sex, Drugs, Ecto-Genitalia (Undertale), Fingering, M/M, Mirror Sex, Mutual Pining, Overstimulation, Penetration, Praise Kink, Recreational Drug Use, Rough Sex, Sensitive bones, Sexual Tension, Size Difference, Size Kink, Voyeurism, aphrodesiac, kingdings
Language: English
Status: Completed
Published: 2019-01-07
Updated: 2019-01-07
Packaged: 2019-10-06 03:19:02
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,748
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/17337611
Author URL: https://archiveofourown.org/users/SynfulCynical/pseuds/SynfulCynical
Summary: After the CORE finally becomes operational, Asgore invites Gaster for a drink in his private chambers to celebrate. The King expresses his desire for something more than a close friendship with the Underground's brilliant scientist. Together, they explore each other's anatomy and desires.





	At His Mercy

**Author's Note:**

  * For [UntoldMuse](https://archiveofourown.org/users/UntoldMuse/gifts).



> Back up secret santa gift for UntoldMuse. I hope it is to your liking. I apologize profusely for the wait.
> 
> Further edits to come. I wanted to get this out to them since it's been overdue. I feel like I'm forgetting tags, but it's 7 am in the morning and I have yet to sleep. More tags to come.

           When the champagne bottle popped open, it only amplified Gaster’s elation and relief. The golden drink foamed in the crystal glasses as the King poured for both of them. Asgore had insisted on a celebratory drink in his private chambers to mark the joyous occasion of the Core finally becoming fully operational, and the brilliant doctor found himself unable to refuse. How could he? After all the sleepless nights and countless hours of work, Asgore proved to be a steady and constant source of support. He had initially planned on returning to his work and side projects after he received high honors from the King, but he supposed a night off would be inconsequential in impact to his schedule. Once the proclamation and ceremony had come to an end, they discreetly slipped away with ample minutes in between each other’s departures to not draw suspicion. The invitation wasn't only for a private drink among friends after all. Their time alone together was sorely overdue.

            Now that all the hard work was behind him, Gaster had eyes only for the King. The doctor found he couldn’t ignore how Asgore’s royal purple, formal robes hung on his broad shoulders. Even as the King passed Gaster a glass, there was graceful restraint in his  movements. The crystal, delicate and sparkling in the bright light, looked all the more fragile in Asgore’s large hand. Gaster could never express how he desired for Asgore to forgo restraint and overpower him in bed. It was not his place, despite the occasional affections and flirtings he received from the King and how close they’d grown during Asgore’s despair over the Queen’s leaving. No, it would be much too bold for him, a subject of the King, to ask for intimacy. Not to mention, he could never replace the Queen, nor would he dare try. It was because he knew how painful a blow her disappearance was to the crown that he kept silent.

           “A toast to your successes, old friend,” Asgore said softly with a warm smile. “And hopefully with many more to come.” 

           They clinked their glasses together, and a clear chime filled the silence between them as they drank.

           “Thank you, Your Majesty,” Gaster said with a slight bow of his head and a shy grin, “but it would never have come to be if it weren’t for you.”

           “Nonsense, Gaster,” Asgore chuckled, placing a hand on the doctor’s shoulder. “And I do trust you know by now when to drop formalities.”

           Gaster nearly flinched at the King’s warmth sinking through his clothes and into his shoulder. He was using every ounce of concentration available to stave off his blush, but he could feel his magic rising to his cheekbones nonetheless. “Of course, Your—Asgore. You know what they say about old habits,” he laughed lightly.

           “Such habits have yet to experience my patience,” Asgore teased. He slowly stroked his thumb along the doctor’s collarbone.

           Gaster’s breath hitched. The glass nearly slipped out of his hand, and the drink sloshed high against the side. He broke his gaze to watch the stream of bubbles rising to the champagne’s surface. Did he miscalculate what all those golden flowers Asgore had given him meant? He hadn’t expected Asgore to make such a subtle, yet forward, gesture for something more. It left him craving Asgore’s touch so intensely, it was maddening as his mind buzzed with electricity and his soul fluttered helplessly like butterflies dressed in spider silk. Embarrassing to say, he felt out of practice. For so long, he buried himself in his work, surrounded by calculations and theories. There was simplicity and logic in a light wave and the conservation of energy, but relationships? Those were more complicated.

           “Have I overstepped a boundary?” Asgore asked. He drew his hand away and looked over Gaster with concern.

           “Goodness, no!” Gaster said, snapping his head up to meet the King’s eyes and placing a hand on Asgore’s chest. “It’s just been a while, and I never imagined...” The doctor’s voice faltered when Asgore coiled an arm around him to draw him closer and pressed a firm hand against his lower back. Strangely, the King seemed taller than he remembered.

           “I see. Perhaps if I phrase it this way…” Asgore began, laughing quietly at having caught Gaster off guard. He leaned over to whisper to him. “I’ve grown very fond of you,  _ Doctor _ , and I would be honored if you let me treat you tonight, especially after all of your hard work.” Asgore was toying with him by addressing him with his title. His smile could be heard accompanying the bass in his voice.

           Gaster shivered at the King’s hot breath falling against his neck vertebrae. He whined softly when the cool air replaced the quickly fading heat. His hand was tingling from feeling Asgore’s voice rumbling deep in his chest. It was as if he pressed his hand to a windowpane and felt the distant roar of thunder. The magic in his bones prickled in anticipation so that the tingling in his hand traveled up his arm and nested behind his ribs. Gaster gripped Asgore’s silken robe in apprehension as he struggled to ask for what he’d been wanting, even when the King had given him permission and expressed a desire of his own. 

           “Would you like to help me with an experiment of mine?” Gaster asked hesitantly in a voice that was barely above a whisper. He didn’t dare to look at the King, not when his cheeks were already burning from embarrassment and his bones were on the verge of rattling. He felt silly for phrasing it this way, but it was the only context he felt comfortable with. Besides, he had been meaning to conduct this one trial in particular.

           “What do you have in mind?” Asgore exhaled heavily, taking delight in seeing Gaster shudder once again.

           “Something I have been working on in my spare time. An aphrodisiac, if you will,” Gaster smiled sheepishly. He reached into his jacket and pulled out a vial filled with what looked like small, white pearls. He popped the cap open with one hand and tipped it over his glass so a single pearl fell out. The champagne immediately fizzed with a blizzard of bubbles until the pearl completely dissolved. “May I?” he asked, gesturing to the King.

           “Please,” Asgore growled in pleasure. He watched one drop like a stone into a sunrise stained pond, the splash and ripples giving way to effervescence.

           Gaster capped the vial and pocketed it away. He held his glass up to the light and tilted it to examine for any signs of the pearl. “Excellent,” he breathed in satisfaction. The change in focus helped to ease his anxiety. “Dissolving to completion is key. I also find alcohol to be a great enhancer for its effects. It is my hope that this aphrodisiac will help boost our populations, but it is still in the testing phases for potency.” The doctor froze when he heard Asgore chuckle warmly.

           “I love how excited you get about your experiments and inventions, Doctor.” Asgore said, raising his glass while looking over Gaster with intense mirth and pride. “And I look forward to being mentored by you since I have little experience with a body type like yours.”

           Gaster’s blush deepened. Come to think of it, he hadn't much experience with the King's body type either. Still, he had faith in his ability to adapt, though he suspected the King being well endowed would require a bit more trial and error on his part. “To new experiences?” he asked, raising his glass.

           Asgore nodded. Together, they finished off the rest of their champagne and set the glasses down on the table.

           Immediately, Gaster felt his soul pulsate and his core flood with magic that quickly spread to the magic lines in his spine and limbs. His bones tingled with a pleasant warmth that heightened his sense of touch and eased the tension in his body. How lofty with euphoria his thoughts were, and his body seemingly as light as a young bird’s down! By the sound of Asgore’s intrigued hum, the King was feeling similar amorous effects. As Gaster slipped off his jacket and fumbled with the buttons of his shirt in his blissful state, a soft thump on the floor told him Asgore had shed his robes. Gaster’s eagerness was blossoming expectantly for Asgore’s command, and he vowed on his soul to follow every one. When the last button came undone and he had unbuckled his belt, Asgore pulled him away from where he stood.

           Asgore coiled an arm around Gaster’s ribs from behind. He pressed his body against him and gently explored the spaces in between bones, while his other hand tightly held the skeleton’s wrist. If Gaster had flesh, Asgore’s strength would have been enough to leave bruises come morning. It was dizzying being caught in the balance between the King’s mercy and power. As the heat from Asgore’s body sank into Gaster’s bones, a tantalizing thought of having to conceal marks crossed the doctor’s mind.

           His imagination of that potential thrilling game was sideswiped when he felt what must have been the King’s cock grinding against him. He gasped sharply, then moaned when the magic in his pelvis tightened and began to condense. The magical slit that was forming throbbed dully in anticipation to take in every inch of the King. He wanted to scream for Asgore to take him then and there, but he knew he wasn’t ready yet despite how his body trembled and his breath quickened.

           It wasn’t until his hand braced against the glass that Gaster noticed the mirror in front of him. His face flushed at the sight of himself, shirt dishevelled and half hanging off of his body while Asgore caressed his ribcage. Every time Asgore moved, his fur grazed against the sensitive gaps between Gaster’s ribs like a soft, watercolor brush. The doctor groaned, long and low, against the mirror so that his hot and heavy breath condensed against the cool glass. The begging expression he wore as Asgore stroked the underside of his ribs and sternum made the magic in his pelvis tighten with slickness. It was a stark contrast to his usual meticulous and tidy appearance, and in any other setting he would have been ashamed of letting himself be seen in that state.

           “You look divine, my dear Doctor,” Asgore whispered as he gazed at their reflection, his mischievous smile visible in his eyes. The drug had brought out the pent up lust that had lied dormant for so long. He nuzzled into the side of Gaster’s neck, then replaced his affections with kiss after fervent, burning kiss that told of his loneliness and desperation. It was a story Gaster knew all too well. He was there for the sleepless nights that smelled of coffee, the heaving sobs of anguish and despair among broken glass from picture frames, and the heartache that manifested as bitterness in tea brewed for too long.

           “A-Asgore…” Gaster said breathlessly as he trembled in the King’s hold. Lost in a daze, he was impressed with Asgore’s remarkable work thus far, although he supposed the King’s long and illustrious experience was an advantage when it came to encountering different biologies.

           Asgore held Gaster tightly against him as he reached for the zipper on the doctor’s pants. He chuckled at the nervous shiver that rippled through the other’s body. The metal teeth parted smoothly with a slow and satisfying zip. Pulling the remaining garments down, he set his eyes on the condensed magic’s reflection. “How fascinating,” he said in wonder, staring at the shadow-like construct that looked almost cosmic in nature. “Tell me, Doctor,” he whispered playfully, “how should I proceed?”

           Had his physiology been different, Gaster’s mouth would have run dry. His voice sounded hoarse nevertheless, and his skull burned hot as he stared at the reflection of his own manifested anatomy. “I-it’s quite simple, really,” Gaster voice quivered. The King teasingly hovered his hand by his ilium, but it was close enough for Gaster to feel the radiating heat and make his bones prickle in response. “The ilium possesses multiple structures that are erogenous for my kind. S-some places in particular are the wings of ilia, also known as the hip bones. Light c-contact on the iliac crest, or the outer edge of this wing, is pleasant—”

           Without hesitation, the King rubbed his thumb lightly along the wing’s edge. Gaster squirmed in shock, back arching as he tilted his head away from his reflection. The high-pitched moan that sprang from his mouth encouraged Asgore to encroach away from the crest and press further in. It wasn’t long before his whimpers blossomed into a climbing whine, like vibrato on a tense violin string, when Asgore massaged slow circles into the wing. If Asgore hadn’t been holding him so securely, he would have collapsed like a marionette with its strings cut. Clenching his jaw shut, his head rolled to one side against Asgore’s chest. It was torturous to stare at his own reflection, to watch himself frozen in a moment of glass like a splayed insect encased in amber, but he found he couldn’t look away as he watched himself come undone. He desperately wanted to feel the King inside of him. He didn’t know how much more he could hold out before he was forced to scream for it.

           “I’m glad to see you enjoying yourself,” Asgore said playfully in between kisses he lovingly placed against Gaster’s neck and cheekbone. His lips didn’t feel as scorching as they did before.

           “I need you, Asgore” Gaster begged softly. “Please, sire!”

           Asgore smiled wryly at Gaster’s uncharacteristic impatience. “In due time, Doctor. I do not wish to hurt you,” he teased as he moved to feel the manifested slit, running a finger along the opening. Slowly, he pushed a finger inside and rubbed against the walls curiously. He hummed in amusement at the whimper that followed. “But perhaps I should prepare you.”

           Gaster let out a strangled moan that was punctuated by his gasps and quivering pleadings when a second finger was added. He held onto Asgore’s restraining arm while his other hand reached for the King’s exploring fingers, only to weakly overlap his grasp on the king’s wrist. He was being rhythmically stretched out, at first in a gentle manner that soon grew more intense the longer the king pressed.

           “Intriguing. Never would I have guessed your body could accommodate me.”

           “My k-kind is very versatile when it concerns pleasuring others,” Gaster managed to say in between ragged breaths. “Much of my body is composed of magic that does not fall under the spectrum of visible light, and only can be seen when condensed to create physical structures. This magic is a lot more...pliant.” His breath caught when a third finger was added. He cried out in ecstasy when Asgore pressed against a sensitive spot that was unknown even to him. Shutting his eye sockets tightly, he saw bursts of color in the dark, like a patchwork of supernovas stitched into his consciousness. If he wasn’t ready before, he certainly was ready now.

           A deep and primal growl of pleasure rumbled in Asgore’s chest when he heard Gaster cry out. He tore off the remaining, hanging garments from Gaster’s body and lifted the doctor to carry him to the bed. Gaster was set down roughly on his back, and he gasped sharply when the King spread him wide open. He couldn’t remember the last time he was so exposed, but he was aware of his soul pulsing hard against his ribs. It left him breathless and exhilarated at the prospect of the King taking—no, claiming what was rightfully his, and Gaster again vowed to follow Asgore’s every command forevermore.

           As a final act of taunting restraint, the King positioned himself to impale Gaster on his cock, but held off. He pinned the doctor’s arms down by the wrists, then planted kisses on Gaster’s collarbone and ribs to mark him. Gaster found himself caught in the balance again, his wrists withstanding against Asgore’s ironclad grip while merciful kisses washed over him. Never would he have guessed Asgore had a playful nature to him, let alone for it to be so maddening.

           “Just fuck me now, Asgore, please!” Gaster begged loudly.

           “A pleasure for a wonderful instructor,” Asgore praised, smiling at the deepening blush that spread across the doctor’s face. He teased the slit with his head, which immediately incited Gaster to protest until he pushed his throbbing cock against the other’s entrance. 

           There was a unity of pleasure and pain in the pressure that built up in Gaster’s pelvis.

The King wrapped an arm around him, fingers tracing the ridges in the doctor’s lower spine as he thrusted, pushing deeper inch by inch. Gaster moaned through clenched teeth, arching his back from fingers charting his vertebrae, his wet folds fluttering and clinging to Asgore’s cock. It wasn’t even the full length and he was struggling to accommodate the King. By the sound of the King’s grunts, and the jerking thrusts that followed, Asgore was in ecstasy over how Gaster’s slick magic contracted tightly in pulses around his length.

           Gaster dug his fingers into Asgore’s muscular back, clawing desperately until his magic finally relented with a faint, dull pop that sent streaks in his vision like comet trails in the night. When the King was fully hilted inside, his cock pushed hard against that sensitive wall, causing Gaster to wail and whimper loudly. The doctor clung to Asgore, his breath ragged, hips gyrating to match the other’s rhythm. 

           “You’re magnificent,” Asgore panted in awe, his hot breath falling against Gaster’s neck. He cupped the side of the doctor’s face, his thumb stroking the other’s cheekbone, and pressed his lips against parted teeth. Instinctively, Gaster’s magic formed a tongue to meet Asgore’s, and the King moaned at the contact that was akin to fire easing winter’s lingering chill. This was a choosing kiss to mark Gaster, that the King had claimed him for himself, that this was gratitude for loyalty and love. 

           Faint and breathless, Gaster mewled softly at the King’s praise and soft lips, but what bound him to delirium was his helplessness as Asgore weighed down on top of his smaller frame. His magic coiled tightly like a whip eager to crack like lightning, rippling pleasure across his bones so they flushed faintly with color. Together, they balanced precariously near the edge. How far they had yet to fall, their arms entangled around each other and their futures now tethered by gravity.

           The King massaged concentric circles of galaxies destined for collision into Gaster’s hip, and it was all it took to make Gaster cum hard. The doctor’s pleasure tore through him like an erupting solar flare, he swore the beginning of the cosmos was inside him. “My King!” Gaster wailed, his voice breaking into a scream as he rode the peak of his orgasm.

           That seized Asgore’s attention. The King’s hard cock continued to ram into him with no restraint, swelling in the climax, it was all nearly too much. Gaster’s oversensitive magic pulsed rhythmically in a frenzy, causing him to plead incoherently in a voice that sounded light years away to himself. His mind was revolving in a daze, but he was aware enough of the pooling warmth that filled him wholly. Asgore’s caressing hand was hot against his face, and he nuzzled his cheekbone into the King’s palm to ground himself all the while whimpering softly. With a thump, his arms fell at his sides in incredible weakness, and he wondered what magic remained to keep his bones strung together.

           Finally, the King collapsed over him, his breaths long and ragged. He took a moment to collect himself before slowly pulling out. He gently hushed Gaster when the doctor’s breath hitched into a tortured mewl, cooing his praises in between kisses along Gaster’s neck and collarbone.

           Gaster heard a soft pop that preceded the jarring emptiness he felt. His magic quivered and throbbed in response as his wet folds collapsed and slowly evanesced.  Chest heaving as he gasped for breath, he wheezed when Asgore gingerly embraced him and held him close. He still could barely stand Asgore’s touch, but as the King stroked his skull and his overstimulated bones began to cool, he could feel himself coming down from his high. He allowed himself to relax in the King’s strong arms, his groans softening into silence.

           “You’re alright, old friend,” Asgore continued to coo, the main effects of the drug now fading like a fog burnt away by dawn’s light.

Gaster hadn’t the voice to speak. The only sounds to come from him were his shallow breaths as he stared with unfocused eyelights at the ceiling. He shivered when Asgore’s whispering breath brushed against his neck. 

“That was quite the experiment,” Asgore continued worriedly. He lightly stroked his knuckles along the side of Gaster’s face. Only when Gaster pushed his head into his touch did the King relax.

“I...don’t regret it…” Gaster managed to croak. Awareness gradually returning, he slowly tried to sit up to get an accurate charting of his surroundings, but Asgore anchored him down and pulled the covers over him. Relenting, Gaster groaned and realized the dull aches that were taking root, although they did little to dispel the calming euphoria that consumed him. Whether it was from magic-based endorphins, the aphrodisiac’s after effects, or both, he knew not. There was no doubt, however, he would be aching in the coming days.

           Asgore couldn’t help but crack a grin. “You know, I think your invention is a little too potent,” he chuckled quietly.

           Gaster weakly covered his eyes in embarrassment as he joined in Asgore’s laughter. “Perhaps so,  _ My King _ ,” he teased lovingly in a raspy voice. “But I think I can count on your  _ patronage  _ to perfect it.”

           Asgore smiled wryly, grateful Gaster saw the humor in their encounter. He rolled over so his upper half was on top of him, and he cupped the side of Gaster’s face. “I love you. I’m sorry I haven’t been more clear,” Asgore said in a low voice. He gently pressed his mouth against Gaster’s forehead.

           “It’s alright. I know now,” Gaster said, smiling softly as he overlapped his hand over the King’s.


End file.
